As a method for producing methyl acrylate or methyl methacrylate according to an oxidative esterification reaction using at least one selected from propylene, isobutylene, and t-butyl alcohol as a starting material, and an unsaturated aldehyde as an intermediate, there are known two methods: a method referred to as the “direct metha process” including two reaction steps; and a method referred to as the “direct oxidation process” including three reaction steps. According to “Sekiyu Kagaku Purosesu (Petrochemical Processes)” (edited by the Japan Petroleum Institute, pages 172 to 176, Kodansha Scientific, Ltd.), the direct oxidation process is a process for producing methyl acrylate or methyl methacrylate in three steps. The first oxidation step is a step of subjecting at least one starting material selected from propylene, isobutylene, and t-butanol to a gas-phase catalytic oxidation reaction with molecular oxygen in the presence of a catalyst, to produce acrolein or methacrolein. The second oxidation step is a step of subjecting the acrolein or the methacrolein obtained in the first oxidation step to a gas-phase catalytic oxidation reaction with molecular oxygen in the presence of a catalyst, to produce an acrylic acid or a methacrylic acid. An esterification step is a step of further subjecting the acrylic acid or the methacrylic acid obtained in the second oxidation step to esterification, when using methanol as an alcohol, to obtain methyl acrylate or methyl methacrylate. Meanwhile, the direct metha process contains two catalytic reaction steps, wherein the first reaction step includes subjecting propylene or isobutylene and/or t-butanol as a starting material(s) to a gas-phase catalytic oxidation reaction with a molecular oxygen-containing gas to produce acrolein or methacrolein, and the second reaction step includes reacting the obtained acrolein or methacrolein, for example, methanol as an alcohol, and molecular oxygen, to produce methyl acrylate or methyl methacrylate in one step.
A catalyst found years ago by Sohio Company has existed as a catalyst for producing an unsaturated aldehyde as a main component. Thereafter, many complex oxide catalysts containing Mo and Bi as essential components have been reported. For example, a catalyst focusing on Mo, Bi, Ce, K, Fe, Co, Mg, Cs, and Rb as metals contained in the catalyst is described in Patent Document 1. Catalysts for producing an unsaturated aldehyde and an unsaturated acid are described also in Patent Document 2. Among them, a catalyst represented by Sb0.5Cs0.5S0.25Ni2.5Co4.5Fe4Bi1Mo12Ox provides the maximum per-pass yield.